Although the battle is said to be the most important between the early northern and southern divisions of the Anglo-Saxons in Britain, few details are available. The two armies met on the banks of a river named the "Winwaed", but this river has never been identified. Possibly it was a tributary of the Humber. There is reason to believe it may have been the river now known as Cock Beck in the ancient kingdom of Elmet, which passes Pendas Fields, Leeds, before joining the River Wharfe (which eventually feeds into the Humber).[9] Another possibility is the River Went, a tributary of the River Don, situated to the north of modern-day Doncaster. It could also be in Oswestry or Winwick.[10]

Penda, after gathering allies from East Anglia and Wales, marched with a force led by "thirty warlords".[12] Oswiu, who was Oswald's brother but had succeeded him only in Bernicia, the northern part of Northumbria, was besieged by Penda's forces at a place called Urbs Iudeu (which has been identified, perhaps dubiously, with Stirling[13]) in the north of his kingdom. Iedeu appears as a historic name for Jedburgh, also located in the north of the kingdom.[14] Apparently Oswiu was desperate enough to offer a great deal of treasure to Penda in exchange for peace. Although the sources are unclear, it is likely that some sort of agreement was reached at Iudeu: although Bede says that Oswiu's offers of treasure were rejected by Penda, who, Bede says, was determined to destroy Oswiu's people "from the highest to the lowest", he does mention that Oswiu's young son Ecgfrith was being held hostage by the Mercians, perhaps as part of a deal. The Historia Brittonum contradicts Bede regarding the treasure, saying that Penda distributed it among his British allies, which would presumably mean that he accepted it.[15] The recorded events may be interpreted to mean that Penda and his army then began marching home, but for some reason the two armies met and fought at a place called the River Winwaed. Breeze (2004) argues that Penda and his army would have been in a difficult strategic location along the Went during their withdrawal, giving Oswiu a good opportunity to attack. It is almost certain that the small (perparvus, according to Bede) Northumbrian forces were considerably outnumbered by the Mercians and their allies. According to Bede, before the battle Oswiu prayed to God and promised to make his daughter a nun and grant twelve estates for the construction of monasteries if he was victorious.[15]

Penda's army was apparently weakened by desertions. According to the Historia Brittonum, Penda's ally Cadafael ap Cynfeddw of Gwynedd (thereafter remembered as Cadomedd, "battle-shirker") abandoned him,[16] along with his army, and Bede says that Aethelwald of Deira withdrew from the battle to await the outcome from a place of safety. Penda was soundly defeated, and both he and his ally, the East Anglian King Aethelhere, were killed, with thirty allied leaders of warbands (duces regii).[17] The battle was fought by the river in the midst of heavy rains, and Bede says that "many more were drowned in the flight than destroyed by the sword". Bede mentions that Penda's head was cut off. Writing in the 12th century, Henry of Huntingdon expanded his version of Bede's text to include supernatural intervention and remarked that Penda, in dying violently on the battlefield, was suffering the same fate he had inflicted on others during his aggressive reign.[18]

The battle had a substantial effect on the relative positions of Northumbria and Mercia. Mercia's position of dominance, established after the battle of Maserfield, was destroyed, and Northumbrian dominance was restored; Mercia itself was divided, with the northern part being taken by Oswiu outright and the southern part going to Penda's Christian son Peada, who had married into the Bernician royal line (although Peada survived only until his murder in 656). Northumbrian authority over Mercia was overthrown within a few years, however.[19]

Significantly, the battle marked the effective demise of Anglo-Saxon paganism; Charles Plummer, in 1896, described it as "decisive as to the religious destiny of the English".[20] Penda had continued in his traditional paganism despite the widespread conversions of Anglo-Saxon monarchs to Christianity, and a number of Christian kings had suffered death in defeat against him; after Penda's death, Mercia was converted, and all the kings who ruled thereafter (including Penda's sons Peada, Wulfhere and Æthelred) were Christian.[21]

^Manuscript A of the Anglo-Saxon Chronicle gives the year as 655. Bede also gives the year as 655 and specifies a date, 15 November. R. L. Poole (Studies in Chronology and History, 1934) put forward the theory that Bede began his year in September, and consequently November 655 would actually fall in 654; Frank Stenton also dated events accordingly in his Anglo-Saxon England (1943).[2] Others have accepted Bede's given dates as meaning what they appear to mean, considering Bede's year to have begun on 25 December or 1 January.[3] The historian D. P. Kirby suggested the year 656 as a possibility, alongside 655, in case the dates given by Bede are off by one year.[4] The Annales Cambriae gives the year as 657.[5]

^Archaeologia Aeliana, or, Miscellaneous Tracts Relating to Antiquity by Society of Antiquaries of Newcastle upon Tyne Published by Society of Antiquaries of Newcastle upon Tyne, 1857 Item notes: ns.1. Original from Oxford University. Digitized 24 January 2007

^Bogg, Edmund (1904). The old kingdom of Elmet, the land ʻtwixt Aire and Wharfe. York: Bogg. p. 135. OCLC4567990.

^Prestwich, J. O. (1968), "King Æthelhere and the Battle of the Winwaed", The English Historical Review, 83 (326): 89–95, doi:10.1093/ehr/lxxxiii.cccxxvi.89, restores to Penda Bede's phrase auctor ipse belli, the "author of the conflict himself", which some scholars had applied to Aethelhere.